In the description which follows, reference is made primarily to the eye and the anterior chamber of the eye as the object. This is to be understood as merely exemplary to assist in the description and not as a restriction. Where the term “eye” is used, a more general transparent and scattering object or organ may be sought instead.
Low coherence interferometry is an absolute measurement technique which allows high resolution ranging and characterisation of optoelectronic components as presented in the papers S. A. Al-Chalabi, B. Culshaw and D. E. N. Davies, “Partially coherent sources in interferometric sensors”, First International Conference on Optical Fibre sensors, 26-28 Apr. 1983, I.E.E. London, pp. 132-135, 1983, R. C. Youngquist, S. Carr, and D. E. N. Davies, “Optical coherence-domain reflectometry: A new optical evaluation technique,” Opt. Lett. 12(3), pp. 158-160 1987 and H. H. Gilgen, R. P. Novak, R. P. Salathe, W. Hodel, P. Beaud, Submillimeter optical reflectometry”, Lightwave Technol., Vol. 7, No. 8, pp. 1225-1233, 1989.
The first application in the biomedical optics field was for the measurement of the eye length as shown in A. F. Fercher, K. Mengedoht and W. Werner, “Eye length measurement by interferometry with partially coherent light”, Opt. Lett., Vol. 13, No. 3, (1988), pp. 186-189.
Adding lateral scanning to the scanning in depth, allows acquisition of 3D information from the volume of biologic media. This concept, of adding devices for lateral scanning in an interferometer, has been presented in papers on heterodyne scanning microscopy, such as “Optical heterodyne scanning microscope”, published by T. Sawatari in Applied Optics, Vol. 12, No. 11, (1973), pp. 2766-2772 and Profilometry with a coherence scanning microscope”, by B. S. Lee, T. C. Strand, published in Appl. Opt., 29, 26, 1990, 3784-3788. The later report shows a cross section image from a semiconductor wafer proving the possibility for subsurface imaging.
The potential of the technique for high resolution imaging of the tissue is often referred to as optical coherence tomography (OCT) as presented in D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito and J. G. Fujimoto, ‘Optical coherence tomography’, Science 254, pp. 1178-1181, 1991 and in the paper “Optical coherence tomography” by A. F. Fercher, in J. Biomed. Opt., 1(2), (1996), pp. 157-173. OCT has the potential of achieving high depth resolution, which is determined by the coherence length of the source. For example, optical sources, such as superluminiscent diodes and mode-locked lasers are now available with coherence lengths below 20 μm.
An OCT apparatus is now commercially available (e.g. from Humphrey), which produces longitudinal images only, i.e. images in the planes (x,z) or (y,z), where the z axis is perpendicular to the patient's face and x and y axes are in the plane of the patient's face. Examples of such apparatus for longitudinal imaging are described in U.S. Pat. Nos. 5,493,109, 5,537,162, 5,491,524, 5,469,261, 5,321,501 and 5,459,570 (Swanson).
OCT has also been reported as being capable of providing en-face (or transversal) images, as reported in “Coherence Imaging by Use of a Newton Rings Sampling Function” by A. Gh. Podoleanu, G. M. Dobre, D. J. Webb, D. A. Jackson, published in Opt. Lett., Vol. 21, No. 21, (1996), pp. 1789-1791, “Simultaneous En-face Imaging of Two Layers in Human Retina” Opt. Letters, by A. Gh. Podoleanu, G. M. Dobre, D. J. Webb, D. A. Jackson, published in Opt. Lett., 1997, vol. 22, No. 13, pp. pp. 1039-1041, “En-face Coherence Imaging Using Galvanometer Scanner Modulation” by A. Gh. Podoleanu, G. M. Dobre, D. A. Jackson, Opt. Lett. 23, pp. 147-149, 1998 and in “Transversal and Longitudinal Images from the Retina of the Living Eye Using Low Coherence Reflectometry”, by A. Gh. Podoleanu, Mauritius Seeger, George M. Dobre, David J. Webb, David A. Jackson and F. Fitzke, published in the Journal of Biomedical Optics, 3(1), pp. 12-20, 1998 and in the U.S. Pat. No. 5,975,697 (Podoleanu).
As shown in the last paper mentioned above and disclosed in the last patent mentioned above, en-face scanning allows generation of constant depth OCT images as well as cross section OCT images initially reported by using longitudinal OCT.
En-face OCT imaging requires movement of at least one of the transverse scanners in a 2D scanner assembly faster than the scanner performing the depth scanning. To generate a raster looking image, the en-face OCT employs a fast transverse scanner and a slow transverse scanner, both operating faster than the scanner performing the depth scanning. In order to adjust the reference path length, in the papers and patents mentioned above, mirrors are used which are translated by mechanical means. This is characterized by the disadvantage that the signal has to be extracted from single mode optical fiber and reinjected back into the same or a different single mode optical fiber. This procedure introduces losses and requires specialized high accuracy and high mechanical stability 3D stages for launching light into a single mode fiber. In a series assembly line in a factory, such a configuration would require significant assembly time and the final product would be expensive.
Therefore, using an all fiber reference path would be advantageous. An all fiber configuration is disclosed in the U.S. Pat. No. 6,201,608B1. However, this disclosure employs a specialized light source which operates in regime of amplification for the signal returned from the target. Two optical paths in fiber are constructed in combination with the specialized optical source. In this case, the depth scanning is achieved by moving a stage supporting a group of elements towards and backwards from the object. The elements grouped on the moving stage are the transverse scanner and the interface optics only.
Such a grouping and assembly has the added disadvantage that a fiber loop is required to connect the stage with the rest of the OCT system. When the stage moves, vibrations are induced in the fiber loop which leads to noise. Also, the polarization of the light propagating down the fiber link may change due to the alteration in the spatial distribution of radiation within the fiber cord, which leads to reduction in the visibility and signal to noise ratio. Such a solution requires an expensive light source and expensive polarization maintaining fiber in order to avoid noise generation in the fiber and changes in the polarization due to fiber cord being shaped during the depth scanning.
Additionally, as shown in the paper by T. Sawatari mentioned above, in order to generate an interference image in the heterodyne scanning microscopy, a phase or frequency modulator is required to create a bit signal, or a carrier for the image signal. The embodiments in the U.S. Pat. No. 6,201,608B1a use such phase moduostors. Such modulator is expensive, ads losses, reduces the efficiency in using the signal and introduces dispersion which deteriorates the depth sampling profile of the OCT.
As another disadvantage, during the stage movement, the focus position slips away from the coherence gate position, given by the point in the volume of the object, where the optical path difference in the interferometer is zero. The longer the depth scanning, the larger the difference between the focus and the coherence gate point, with disadvantageous reduction in the signal strength.
Signal strength and transverse resolution depend on how well the focus is matched to the coherence position (wherein tracking of the focusing and zero optical path difference are referred to as dynamic focus). Dynamic focus was described in PCT patent publication No. WO 92/19930, but only in principle. Possible optical configurations to simultaneously scan the depth and the position of the focus in the depth are described in U.S. Pat. No. 4,589,773, in U.S. Pat. No. 6,057,920 and in U.S. Pat. No. 6,144,449. These solutions however, require mechanical synchronism of elements or adjustment of ratios of focal lenses or movement of a bubble elastic lens respectively, with the consequence limitation in speed.
Another method was described in the paper “An optical coherence microscope with enhanced resolving power in thick tissue”, by J. Schmitt, S. L. Lee and K. M. Yung, published in Optics Communications, 142, (1997), pp. 203-207 where the focusing lens in the object arm was synchronously moved with retroreflectors in the reference arm. In this way, for a movement of the objective lens towards the tissue by x, the OPD varies by 2n2x−4, where n is an average value for the index of refraction of the medium. When n2 is approximately 2, which happens for most of the tissue structures, then OPD is approximately zero and dynamic focus is automatically accomplished. However, the method uses a mirror which redirects high power to the optical source, and it is known that low coherence source are prone to noise in the presence of feedback. The movement employs elements in both object and reference arm which makes the method cumbersome to implement. Another method for dynamic focus applicable for the case when n2 is approximately 2 is disclosed in the patent WO 02/04884. This last disclosure presents a simultaneous movement of a lens and of a beam-splitter separating the reference and the object beams in the interferometer. This requires stable mechanical fixtures, low vibrations and the method cannot be implemented in fiber version.
The dynamic focus methods described above are devised especially for longitudinal OCT, where B-scan images are generated by fast scanning along the depth coordinate with a slower scanning along a transverse coordinate. As such, the method needs to be fast, and operational at the depth scanning rate of, for example, a rate on the order of 100-1000 Hz.
The U.S. Pat. No. 6,201,608B1 provides for solutions for dynamic focus to maintain the coherence gate in synchronism with the focus dedicated for microscopy. The solutions presented are restricted to high numerical aperture values, 0.4 to 1.5, as required by microscopy applications of imaging small objects. Even if no dynamic focus is applied, if the specimen thickness is small, such as the case in microscopy, scanning the depth using a translation stage as that described in FIG. 8a is feasible. However, a system using the implementation in FIG. 8a cannot be used to image large curved objects which require axial scans much larger than the thickness of the tissue investigated. A typical example is that of the cornea, which has 0.5 mm thickness, but axial scanning has to cover 5 mm to 10 mm. Such an object to be imaged requires a solution of dynamic focus. The U.S. Pat. No. 6,201,608B1 does not show how such large objects could be imaged by moving a stage with the scanner and the interface optics axially to and back from the object. As another problem when imaging the cornea is the high signal reflected from the epithelium with diminished brightness for the scatterrers from inside the cornea tissue. In many imaging applications the exact ratio of brightness from different pixels is less important as to collect sufficient signal image to display the morphology from deep in the tissue. If dynamic focus was applied to the cornea, then the high signal from the epithelium may saturate the electronics circuitry.
Thus, a need exists for a better procedure of implementing the depth scanning and processing of the OCT signal. In particular, in the first instance, a better configuration less susceptible to noise and which does not alter the polarization state would be desirable. Secondly, a procedure having improved efficiency in using the signal and tolerant to dispersion would be advantageous. Thirdly, a procedure to implement axial scanning by moving the transverse scanner and the interface optics to obtain sufficient signal collection from large and curved objects without dynamic focus. Fourthly, a solution for dynamic focus to maintain at least partially, the synchronism between the focus and the coherence gate points during the depth scanning would be desirable.
Accordingly, the present invention provides for improvements over at least one of the problems of the prior art as stated hereinabove, or as described herein below.